A photonic network having an optical add-drop multiplexer and/or a wavelength crossconnect has been proposed and developed. The optical add-drop multiplexer (ROADM: Reconfigurable Optical Add/Drop Multiplexer) is capable of dropping an optical signal of a desired wavelength from a WDM optical signal and guiding the dropped signal to a client, and capable of adding a client signal of any wavelength to a WDM optical signal. The wavelength crossconnect (WXC: Wavelength Cross Connect or PXC: Photonic Cross Connect) is capable of controlling the route of an optical signal for each wavelength, without converting the optical signal into an electric signal.
In a photonic network as described above, a plurality of optical paths (here, wavelength paths) that use the same wavelength may be set. For this reason, in order to establish and operate a network certainly, for example, a scheme to superimpose a path ID to identify each optical path on an optical signal has been proposed. In this case, optical node equipment (here, the optical add-drop multiplexer, the wavelength crossconnect and the like) has a function to detect the path ID superimposed on the optical signal. Accordingly, since each optical path can be identified certainly at the optical node equipment, it becomes possible to monitor/detect/avoid a failure such as to connect the optical fiber to a wrong port, and so on.
As a technique to manage the optical path, a method having the following steps has been proposed. The steps includes combining at least one payload data stream with at least one side data stream comprising the path ID into a composite electrical data stream; applying the composite data stream to an optical transmitter to produce an optical signal; detecting the optical signal with an optical receiver having a maximum frequency of operation less than one-half of the rate of the composite data stream; and recovering the side data stream from the electrical output of the optical receiver. (for example, U.S. Pat. No. 7,580,632).
In addition, related arts are described in U.S. Pat. No. 7,512,342, US Patent Publication No. 2009/0169210, US Patent Publication No. 2010/0080568, Japanese Laid-open Patent Publication No. H11-331224, Japanese Laid-open Patent Publication No. 2008-263590, Vinay A. Vaishampayan and Mark D. Feuer, “An Overlay Architecture for Managing Lightpaths in Optically Routed Networks,” IEEE Transactions on Communications, Vol. 53, No. 10, October 2005.
In a conventional art (for example, FIG. 2 and FIG. 3a in U.S. Pat. No. 7,580,632 and the like), the signal representing the path ID (hereinafter, a path ID signal) is superimposed on the optical signal by, for example, intensity modulation. In this case, cross gain modulation occurs by an optical amplifier that amplifies the WDM optical signal collectively (for example, EDFA) and/or by induced Raman scattering in the optical fiber. The cross gain modulation may induce crosstalk of the path ID signal between wavelength channels in the WDM optical signal. As a result the path ID may be identified wrongly in the optical node equipment.
In another conventional art, after modulating a data signal using a code corresponding to the path ID in the electric domain, an optical signal is generated by optical modulation by the modulated data signal. In this case, for the optical receiver, an optical demodulator corresponding to the optical modulation scheme needs to be provided on the input side of a converter to convert the optical signal into the electric signal. Therefore, in a system in which a plurality of optical modulation schemes are used, a plurality of optical modulators need to be provided, increasing the circuit size. In addition, when the payload data have different symbol rates, it is difficult to collectively adjust the clocks of respective wavelength channels.